Currently, a two-stage inverter circuit structure is usually used in an AC (Alternating current)-DC (Direct Current) argon arc welding machine in the market, as shown in FIG. 1, a square wave AC with low voltage and high frequency is outputted after a voltage signal of the grid is rectified and filtered, is firstly inverted, and then is decreased through a high frequency transformer; and then a DC or an AC welding current having a lower frequency is outputted, after the square wave AC is further rectified and secondary inverted.
A rectifying and secondary inverter circuit part after voltage decrease of a high frequency transformer in the prior art has two types, one type is a mode of a full-wave rectification circuit and a second full bridge inverter circuit, as shown in FIG. 2, a low voltage high frequency AC passes through a full wave rectification circuit composed of a FRD (Fast Recovery Diode) D1, a FRD D2 and is converted into a low voltage DC, the low voltage DC continues to pass through a full bridge inverter circuit composed of VT1, VT2, VT3, VT4 and is converted into a low voltage DC or a low voltage low frequency AC, and the low voltage DC or low voltage low frequency AC is outputted by electric wire and is used for welding. Another type is a mode of a bridge type rectification circuit having a centre tap and a secondary push-pull inverter circuit, as shown in FIG. 3, a low voltage high frequency AC passes through a bridge type rectification circuit composed of FRDs D1, D2, D3, D4 and is converted into a low voltage DC, and then the low voltage DC passes through a push-pull inverter circuit composed of IGBT (Isolated Gate Bipolar Transistor) Q1, IGBT Q2 and is converted into a low voltage DC or low voltage low frequency AC, the low voltage DC or low voltage low frequency AC is outputted through the electric wire and is used for welding.
There are several problems in the aforesaid two types of rectification and secondary inverter circuits in the prior art in an actual application. Taking an electric circuit structure shown in FIG. 2 as an example, in the actual application, there exists several problems as follows: a first problem is that an ordinary welding machine outputs low voltage high current, when the welding machine works, in a conducting loop, there are three conductive power devices (such as, D1+VT1+VT4, or D2+VT2+VT3, and so on), when an existing power device works, no matter the existing power device is a FRD or a IGBT or a field-effect tube, a voltage drop is always generated (generally speaking, the voltage drop is about 1-2V), the power device has an obvious power consumption especially when it lies in a large current state. In order to solve the problem of high power consumption, in a secondary inverter process, a field-effect tube having a low voltage and a low on-resistance is widely used; however, there is a defection that the cost is high and a voltage protection circuit of the field-effect tube is more strictly required; moreover, aiming at a special characteristic of an electric arc welding workpiece, using of a low voltage field-effect tube is not good for electric arc maintenance when there is a current direction change, and an arc interruption phenomenon is prone to occur, thereby affecting a welding effect. A second problem is that, when there is the current direction change, in order to improve an arc stabilization effect, an optimized arc stabilization effect can be obtained when a voltage for arc stabilization is set to be about 250V; when a field-effect tube having a withstanding voltage less than 250V is used, an arc stabilization device is more complicated and the arc stabilization effect is bad; however, when a field-effect tube having a withstanding voltage more than 250V is used, there is always a much larger on-resistance, thereby resulting in an obvious power consumption. A third problem is that a large number of power devices need to be used, and a wire arrangement of an electric board is complicated. Moreover, when the welding machine outputs a low voltage DC, a condition that two groups of power devices of the inverter circuit part are continuously conductive and two other groups of power devices of the inverter circuit part are closed will be happened; under this condition, it inevitably results that power switches are traversed continuously by high current; therefore, when the electric circuit is designed, the number of the power switches or the capability of over current must be increased, which results in an increase in the cost inevitably, and also results in problems such as an inhomogeneous heat dissipation of a heat sink and a bad heating effect, and so on.
However, there also exists some problems in the bridge type rectification circuit having a centre tap and a secondary push-pull inverter circuit as shown in the FIG. 3 in an actual application, one problem is that when the welding machine works, there are two conductive power devices (a FRD and a IGBT) in the conducting loop, compared with the full wave rectification circuit having the centre tap and the secondary full bridge inverter circuit, the number of FSDs is doubled, the number of the power switches are reduced by half, but the withstanding voltage value is doubled, and thus a whole power consumption is substantially unchanged relatively. A second problem is that since the number of the FSDs is doubled, and the bridge type rectification circuit is used, when a high current occurs, a large space is occupied, and the wire arrangement of the electric board is complicated. A third problem is that a push-pull circuit is applied in the secondary inverter circuit, since withstanding voltage value of the power switches is doubled (generally, a voltage of 600V is selected), the existing welding machine usually adopts an IGBT of high withstanding voltage and heavy current, however, a high saturation voltage drop (1-2V) of this IGBT can results in a high power consumption. A four problem is that when the welding machine is in a DC output state, a condition that one group of power switches in the inverter circuit are continuously conductive and the other group of power switches in the inverter circuit are blocked will be happened, and thus it result inevitably that the power switches are traversed by heavy current. Therefore, when the bridge type rectification circuit having the centre tap and the secondary push-pull inverter circuit is designed, the number of the power switches and the capability of over current must be increased, which results in an increase in the cost inevitably, and also results in problems including an inhomogeneous heat dissipation of the heat sink and a bad heat dissipation effect.
As stated above, there exist some problems including high power consumption, high cost and complicated wire arrangement in all power switching circuits in the existing AC-DC argon arc welding machine.